A compander is defined as "a system for improving the signal-to-noise ratio by compressing the volume range of the signal at a transmitter by means of a compressor, and restoring the normal range at the receiving apparatus with an expandor." (See Electronics and Nucleonics Dictionary/John Markus - McGraw Hill Co. (1966).
The companding technique for noise reduction in communication channels is well known. Compression is applied at the input of a communication channel and expansion follows at the receiving end, thus after noise exposure in the communication channel. This method is used in particular for improvement of the signal-to-noise ratio over a voice circuit so that operation becomes possible over noisy circuits. The method consists in imparting more gain to low-intensity signals, e.g., those most affected by the noise, than to high-intensity signals. This reduces the dynamic range of the signals being transmitted.
The invention proposes to effect noise reduction in a somewhat different communication problem, as explained, in copending patent application Ser. No. 033,063 filed concurrently by Harold B. Shutterly, where audio signals are transmitted as part of the video signals of a standard television system. In such situation, while the audio signals are multiplexed into the blanking periods of the video signals for transmission over video channels, the video channel signal-to-noise ratio is frequently too low for the audio signal. The principal reason for requiring a higher signal-to-noise ratio for audio than for video is to prevent audible noise during low-level portions of the audio signal. According to the present invention, the amplitude of the audio signals is increased to a maximum extent before transmission and then decreased by precisely the same factor after reception. This is made possible by an auxiliary control code transmitted with the audio signal. Decreasing the signal amplitude after reception also decreases any added noise by the same factor, thereby effectively increasing the signal-to-noise ratio of the transmission channel for the low-level signals.